1. FATIGUE • Fatigue = failure under cyclic stress.
• Stress varies with time.
--key parameters are S and sm
• Key points: Fatigue...
--can cause part failure, even though smax < sc.
--causes ~ 90% of mechanical engineering failures. 17

2. FATIGUE DESIGN PARAMETERS
• Fatigue limit, Sfat:
--no fatigue if S < Sfat
• Sometimes, the
fatigue limit is zero! 18

3. FATIGUE MECHANISM • Crack grows incrementally • Failed rotating shaft
typ. 1 to 6
increase in crack length per loading cycle
crack origin
• Failed rotating shaft
--crack grew even though
Kmax < Kc
--crack grows faster if
• Ds increases
• crack gets longer
• loading freq. increases. 19

4. IMPROVING FATIGUE LIFE
1. Impose a compressive
surface stress
(to suppress surface
cracks from growing)
--Method 1: shot peening
--Method 2: carburizing
2. Remove stress
concentrators. 20

5. PROCESSING USING DIFFUSION (1)
• Case Hardening:
--Diffuse carbon atoms
into the host iron atoms
at the surface.
--Example of interstitial
diffusion is a case
hardened gear.
• Result: The "Case" is
--hard to deform: C atoms
"lock" planes from shearing.
--hard to crack: C atoms put
the surface in compression. 21

6. MEASURING ELEVATED T RESPONSE
• Elevated Temperature Tensile Test (T > 0.4 Tmelt).
• Generally, . . . 22

7. CREEP • Occurs at elevated temperature, T > 0.4 Tmelt
• Deformation changes with time.
Adapted from
Figs and 8.27, Callister 6e. 23

8. SECONDARY CREEP • Most of component life spent here.
• Strain rate is constant at a given T, s
--strain hardening is balanced by recovery
stress exponent (material parameter) .
activation energy for creep
(material parameter)
strain rate
material const.
applied stress
• Strain rate
increases
for larger T, s 24

9. CREEP FAILURE • Failure: along grain boundaries.
• Estimate rupture time
S 590 Iron, T = 800C, s = 20 ksi
g.b. cavities
applied
stress
24x103 K-log hr
• Time to rupture, tr
temperature
function of
applied stress
1073K
Ans: tr = 233hr
time to failure (rupture) 25

10. SUMMARY • Engineering materials don't reach theoretical strength.
• Flaws produce stress concentrations that cause
premature failure.
• Sharp corners produce large stress concentrations
and premature failure.
• Failure type depends on T and stress:
-for noncyclic s and T < 0.4Tm, failure stress decreases with:
increased maximum flaw size,
decreased T,
increased rate of loading.
-for cyclic s:
cycles to fail decreases as Ds increases.
-for higher T (T > 0.4Tm):
time to fail decreases as s or T increases. 26